BossEyed
Sorry - I realised after posting that some of my comments could be read in the way you have indeed read them. It wasn't my intention at all to leave the impression that only TPs do T&E work.
Of course, the whole process is heavily dependent on the army of boffins who guide the setting up of the T&E programme - and who then do the post-flight analysis of the results which enables Boscombe (or any other T&E establishment) to pronounce with authority on the outcome. It is their accumulated experience over many years, many aircraft types, many situations which has already been largely negated by the co-location of TPs with the OEUs.
What has also been lost is the breadth and depth of experience of the flight line engineers in dealing with new types or variants. However well-trained, the Service lineys on a new type cannot have that experience.